Method and device for aligning a workpiece on a machine tool table

ABSTRACT

The workpiece or pack of printed circuit boards is fitted to a clamping fixture carried by the table and is machined by a machining head; the head and the table being movable with respect to each other along two coordinate X, Y axes. The aligning method includes the steps of providing the pack with two locating holes; fitting the pack to the fixture; determining the distances between the real positions of the holes and two reference positions; and moving the pack on the fixture to eliminate the distances. The aligning device has an optoelectronic sensor carried by the head to determine the real position of each hole; and a number of motors controlled on the basis of the real position of each hole to move a locating member for locating a first pin on the pack, and to move one bar of a pair of bars for clamping the other pin on the pack.

BACKGROUND OF THE INVENTION

The present invention relates to a method and relative device forautomatically aligning a workpiece, in particular a pack of printedcircuit boards, on a machine tool table.

As is known, a workpiece must be positioned accurately on the worktableof a machine tool, which is achieved by means of locating elementsnormally provided on the workpiece itself. In the case of packs ofprinted circuit boards, each pack features a pair of pins projectingfrom one face of the pack, and which are fitted inside seats on aclamping fixture fitted to the table. The fixture comprises a locatingmember for clamping one of the pins, and a pair of bars for gripping theother pin.

Machining printed circuits calls for a high degree of precision. Forexample, each hole drilled on a drilling machine is about 0.5 mm indiameter and must be centered exactly on the respective circuit pad,which may be about 0.6 mm in diameter. As such, the circuit must bepositioned up to one micron, and the same degree of precision must beguaranteed over the entire surface of the circuit, which is normallyrectangular with sides measuring many centimeters in length.

The positioning precision of the pack clamping fixture depends on howaccurately the fixture and the fixture connection to the table aremachined, and on how accurately the two pins are fitted to the pack.And, since the drilling position is determined by the machine on thebasis of two coordinate axes referred to the table, the coordinate axesof the drilling positions must coincide with those of the table for thepack to be positioned accurately.

Even minor mechanical inaccuracies on the fixture, the fixtureconnection and/or the pins, may result in linear or angular deviation ofthe coordinate axes of the pack with respect to those of the table, thusresulting in unacceptably inaccurate drilling. Known fixtures thereforehave the drawback, on the one hand, of having to be machined andassembled extremely accurately, and, on the other, of being incapable ofcorrecting inevitable inaccuracies in machining and/or assembly.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a highlystraightforward, reliable method and device for automatically aligning aworkpiece on a machine tool, and which provide for eliminating theaforementioned drawbacks typically associated with the known state ofthe art.

According to the present invention, there is provided a method ofautomatically aligning a workpiece on a table of a machine tool, whereinthe workpiece is fitted to a clamping fixture carried by said table andis machined by a machining head; said head and said table being movablewith respect to each other along two coordinate axes; and the methodbeing characterized by comprising the steps of:

providing said workpiece with two locating elements;

fitting said workpiece to said fixture;

determining the distances between the real positions of said locatingelements and two corresponding reference positions; and

moving said workpiece on said fixture to eliminate said distances.

According to the present invention, there is also provided a device forautomatically aligning the workpiece on the table of the machine tool,wherein the workpiece is fitted to said fixture by means of clampingmeans movable on said fixture by means of electric motors; control meansbeing provided to so control said motors as to align said workpiece in apredetermined position.

More specifically, the workpiece is defined by a pack of printed circuitboards, and comprises two locating pins which are fitted to said fixtureby means of a locating member for one of said pins, and by two barsmovable towards each other to grip the other of said pins; said electricmotors driving two actuators for moving said locating member along twocoordinate X, Y axes, and a third actuator for moving one of said barsalong one of said X, Y axes.

The locating member is advantageously carried by a slide traveling alongone of said X, Y axes; said slide being guided by a shift member movedalong a first surface inclined with respect to said X, Y axes; a firstof said bars being moved along a second surface inclined with respect tosaid X, Y axes; said inclined surfaces producing a component of movementof said locating member and of said first bar along the other of said X,Y axes.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 shows a view in perspective of a pack of printed circuit boardsfor machining;

FIG. 2 shows a view in perspective of a multiple-head machine toolfeaturing the pack aligning device according to the invention;

FIG. 3 shows a schematic partial plan view of a fixture for clamping thepack onto the FIG. 2 machine according to a first variation of theinvention;

FIG. 4 shows a schematic, partially sectioned plan view of the FIG. 3fixture;

FIG. 5 shows a section along line V--V in FIG. 4;

FIG. 6 shows an overall diagram of the device according to theinvention;

FIGS. 7A-7C shows a block diagram of the logic circuits of the device;

FIG. 8 shows a flow chart illustrating operation of the device;

FIG. 9 shows a schematic partial plan view of a fixture for clamping theFIG. 1 pack according to a further variation of the invention;

FIGS. 10A-10B shows a partial larger-scale top plan view of a detail inFIG. 9;

FIG. 11 shows a larger-scale section along line XI--XI in FIG. 10A;

FIG. 12 shows a section, to the FIG. 11 scale, along line XII--XII inFIG. 10A;

FIG. 13 shows a section along line XIII--XIII in FIG. 10A;

FIG. 14 shows a section along line XIV--XIV in FIG. 10B.

DETAILED DESCRIPTION OF THE INVENTION

Number 1 in FIG. 1 indicates as a whole a pack of printed circuit boards2 as arranged for machining, which normally comprises drilling andmilling. Pack 1 is substantially rectangular with a short side m and along side M, and comprises an auxiliary bottom board 3 and an auxiliarycover board 4.

Boards 2-4 are connected to one another by a pair of cylindricallocating pins 5 and 6, which are located along the center line A of pack1, at two edges 7 parallel to short side m, and project a given lengthfrom the bottom face of auxiliary bottom board 3. As pack 1 may vary insize within certain limits, the distance between pins 5 and 6 variesaccording to the size of boards 2.

With reference to FIG. 2, number 8 indicates as a whole a machine toolcomprising a number of machining heads 9, each having a tool-holderspindle 10; heads 9 are arranged in two rows on a common carriage 11moved along a coordinate axis X by a numerically controlled servomotor(not shown); machine 8 also comprises a worktable 12 moved along acoordinate axis Y by a further numerically controlled servomotor (notshown); and each spindle 10 is movable vertically along a vertical axisZ, e.g. for drilling, by a third numerically controlled servomotor 15.

Table 12 carries a number of fixtures 13, each for clamping a pack 1 formachining; fixtures 13 are arranged in two rows and associated withmachining heads 9; and each fixture 13 (FIG. 3) comprises two straightparallel bars 14 and 16, which are made of extremely hard material, aremachined accurately, and are carried by two coplanar plates 17 and 18.

Bars 14 and 16 form a gap 19 in which are inserted pins 5 and 6 of pack1, which is indicated by the dash line in FIG. 3. In FIGS. 3 and 4, thetransverse dimensions of bars 14, 16 and gap 19 are enlarged for thesake of clarity. Plates 17 and 18 are fitted to table 12 in any knownmanner, and preferably so that bars 14 and 16 extend parallel to the Xaxis, as shown in FIG. 2.

Plate 17 comprises a locating member defined by a block 21 (FIG. 3)having a V-shaped seat 20 for receiving pin 5. Bar 16 is longer than bar14, so as to also face block 21, and is movable parallel to itselftowards fixed bar 14 and block 21 to clamp pins 5 and 6 of pack 1 ontofixture 13. More specifically, bar 16 is activated by a tubularcontainer 22 made of elastomeric material, and which is deformed bycompressed air to so move bar 16 as to clamp pin 5 against seat 20 andpin 6 against bar 14.

According to the invention, block 21 is adjustable on plate 17 in twoperpendicular directions. More specifically, block 21 is fitted to plate17 by means of a plate 23 (see also FIG. 5), two surfaces of which carrya pair of prismatic cross-shaped guides 24 and 25 respectively engaginga groove 26 formed in plate 17 and perpendicular to bar 14, and a groove27 formed in block 21 and parallel to bar 14. Block 21 carries a screwnut 28 engaging a screw 29 connected to the shaft of a first reversibleelectric step motor 31 carried on an appendix 32 of plate 23.

Plate 23 comprises a portion 33 having an inclined rib 34 engaging aninclined groove 36 formed in a further block 37; and block 37 comprisesa rib 38 engaging a groove 39 formed in plate 17 and parallel to bar 14,and carries a further screw nut 41 engaging a screw 42 connected to theshaft of a second reversible electric step motor 43 carried by anappendix 44 of plate 17.

Bar 14 is adjustable on plate 17 in a direction perpendicular to itself.More specifically, bar 14 is provided underneath with a pair of guides46 parallel to the Y axis and engaging a corresponding pair of grooves(not shown) formed in plate 17; bar 14 comprises an appendix 47, whichin turn is provided underneath with an inclined rib 48 engaging acorresponding inclined groove formed in a block 51; and block 51 carriesa screw nut 52 engaging a screw 53 connected to the shaft of a thirdreversible electric step motor 54 carried by a further appendix 56 ofplate 17.

Each pack 1 of boards for drilling is provided with two locatingelements defined by two holes 57 and 58 (FIG. 1), which are associatedwith, and located adjacent to and the same distance from, pins 5 and 6,so that the line joining the centers of holes 57 and 58 is parallel tothe center line A of pack 1. Each machining head 9 in the front row(FIG. 2) carries an optoelectronic sensor defined by a television camera59 for determining the real position of each hole 57, 58 when this isbrought within the optical field of the camera.

Each camera 59 is connected to a corresponding image-processing logiccircuit 61 (FIG. 6); the four circuits 61 are connected to a camera bus62 interfacing with a logic control circuit 63 for controlling thedevice; and control circuit 63 interfaces with a motor bus 64 to whichare connected a number of logic control circuits 66, each forcontrolling the three motors 31, 43, 54 of each fixture 13. In the caseof the FIG. 2 machine, only four control circuits 66 are provided.

More specifically (FIG. 7), each image-processing circuit 61 comprisesan image digitization block 67, which is connected to a processor 68connected to a RAM memory 69 and controlled by a program recorded in anEPROM memory 71; circuit 61 also comprises a serial interface 72 forconnection to bus 62 (see also FIG. 6); and block 67, controlled byprocessor 68, determines the presence of hole 57, 58, and receives andconverts the read data from camera 59 into numeric data.

On the basis of the numeric data, processor 68 determines the center ofthe sensed hole 57, 58 and the dimensions of the real hole positionalong the X and Y axes; and, by comparison with the reference holeposition represented by the position of table 12 and that of carriage11, the components of the distance between the real hole position andthe reference position are determined and recorded in RAM 69.

Control circuit 66 comprises a microprocessor 73 connected to bus 64 viaa serial interface 74, and a set of three known controllers 76, 77, 78for driving respective motors 31, 43, 54; control circuit 63 comprises aprocessor 79 connected to a RAM memory 81 and controlled by a programrecorded in an EPROM memory 82; and circuit 63 comprises an interface 83for connection to bus 62, an interface 84 for connection to bus 64, andan interface 86 for connection to a numeric control unit 87 of themachine.

Operation of the pack 1 aligning device will now be described withreference to the flow chart in FIG. 8.

Packs 1 (FIG. 1) complete with holes 57 and 58 are first placed onfixtures 13 (FIG. 2) and compressed air is fed into containers 22 sothat each pack 1 is clamped by bar 16 with pin 5 inside V-shaped seat 20of block 21 and with pin 6 clamped against bar 14. Numeric control unit87 (FIGS. 2 and 6) then moves table 12 along the Y axis to bring holes57, 58 of packs 1 in the front row of fixtures 13 into the plane ofcameras 59, and moves carriage 11 along the X axis to align each camera59 vertically with the desired or reference position of the respectivehole 57 adjacent to pin 5 (operation 88 in FIG. 8).

Unit 87 then enables control circuit 63 to control the aligningoperation. Circuit 63 first enables circuits 61 sequentially to receiveand digitize the signals read by cameras 59 (operation 89) and to defineand memorize in each RAM 69 the error components along the X and Y axes,i.e. the components of the distance between the real position of hole 57and the reference position (operation 90).

Unit 87 then moves carriage 11 along the X axis to align cameras 59 withholes 58 adjacent to pins 6 (operation 91); and control circuit 63 againenables circuits 61 to receive the signals read by cameras 59 (operation92) and to record in respective RAM 69 the Y-axis component of thedistance between the real position of hole 58 and the respectivereference position (operation 93).

Finally, control circuit 63 enables processing circuits 61 sequentiallyto transfer the recorded X- and Y-axis components of hole 57 and theY-axis component of hole 58 to respective control circuits 66, which arethen enabled to each control the three motors 31, 43, 54 of respectivefixture 13 in the front row.

On each of the front-row fixtures 13, motor 43, by means of block 37(see also FIG. 4), moves plate 23 according to the Y-axis component ofhole 57 determined by respective circuit 61; motor 31 moves block 21according to the respective X-axis component; and, at the same time,motor 54 moves bar 14 according to the Y-axis component of hole 58.

The centers of the two holes 57 and 58 of each pack 1 are thus perfectlyaligned with the respective reference positions (operation 94). Needlessto say, the movements effected by motors 31, 43, 54 are normallyextremely small to simply correct any positioning errors due toinevitable mechanical tolerances of the component parts.

Unit 87 then moves table 12 to align holes 57, 58 of packs 1 in the rearrow of fixtures 13 with cameras 59 of heads 9 in the front row; machine8 and circuits 61 are then operated by unit 87 and circuit 63 to performoperations 88-93 in FIG. 8; and, finally, control circuit 63 enablescontrol circuits 66 to operate motors 31, 43, 54 of rear-row fixtures 13to perform operation 94. The aligning device therefore provides foralternately aligning packs 1 in the two rows of fixtures 13.

FIGS. 9-14 show a pack 1 clamping and aligning fixture 113 according toa variation of the invention, and which comprises two coplanar plates117 and 118, each having a top surface 115 for supporting pack 1.Fixture 113 also comprises two bars 114 and 116 carried by plates 117and 118 and having two opposite straight parallel surfaces 95 and 96.

Bars 114 and 116 are made of extremely hard material, are machinedaccurately, and have respective top surfaces flush with the top surfaces115 of plates 117 and 118; surfaces 95 and 96 define a gap 119 in whichis inserted pin 6 of pack 1, which is indicated by the dash line in FIG.9; and plates 117 and 118 are fitted to the table so that surfaces 95and 96 of bars 114 and 116 are parallel to the X axis.

Plate 117 comprises a locating member defined by a block 121 having aV-shaped seat 120 for receiving pin 5. Bar 116 is longer than bar 114,so as to also face block 121, and is movable parallel to itself towardsfixed bar 114 and block 121 to clamp pins 5 and 6 of pack 1 onto fixture113.

More specifically, bar 116 is activated by a tubular container 122 madeof elastomeric material, and which is deformed by compressed air to somove bar 116 as to clamp pin 5 against seat 120 and pin 6 against bar114. Both block 121 and bar 114 are fitted adjustably to plate 117. Forwhich purpose, plate 117 comprises two cavities 97 and 98 accessiblefrom the top, and each of which is closed by a respective plate 99, 100fitted removably to plate 117 by means of screws 101.

Block 121 is movable inside cavity 97 (FIG. 10) in two perpendiculardirections. More specifically, block 121 is fitted by means of screws108 inside a cavity 109 (see also FIG. 11) of a substantiallyparallelepiped slide 123, which comprises a cavity 110 housing a pin 111for fastening plate 99 by means of a further screw 112.

Slide 123 comprises a flat wall 127 parallel to the X axis and thereforeto surface 95, and which, by means of a return spring 126, rests againsta wall 125 also parallel to surface 95. Wall 125 is carried by aprismatic, trapezoidal-section, intermediate shift member 133, a wall134 of which, opposite wall 125, is inclined with respect to wall 125 bya given angle a and, by means of a second return spring 135, restsagainst a vertical wall 136 of cavity 97. Springs 126 and 135 are housedin three cavities 124, 130 and 140 respectively formed in slide 123, inintermediate member 133 and in wall 136 of cavity 97.

Wall 136 forms a first inclined surface sloping with respect tocoordinate axes X, Y, provides for guiding the movement of member 133,and is so inclined that the movement of intermediate member 133 alongwall 136 itself produces a given component of movement of block 121along the Y axis. More specifically, wall 136 and the X axis form saidangle α, which may range between 5° and 10°.

Slide 123 carries a screw nut 128 (see also FIG. 12) engaging a screw129, which is connected to the shaft of a first reversible electricmotor 131 carried by an appendix 132 of intermediate member 133. Screw129 and the shaft of motor 131 are parallel to the X axis. Intermediatemember 133 carries another screw nut 141 engaging a screw 142 (see alsoFIG. 13) connected to the shaft of a second reversible electric motor143, which is carried by a block 144 fitted to plate 117 by means of twoscrews 145. Screw 142 and the shaft of motor 143 are parallel to the topsurface 115 of plate 117 and to wall 136 of cavity 97.

Bar 114 comprises a wall 147 opposite surface 95 and forming withsurface 95 a predetermined angle, which may advantageously be equal toangle α. Bar 114 is housed in a depression 139 on plate 117 (see alsoFIG. 14), and depression 139 has a vertical guide wall 138 forming asecond inclined surface at angle α with respect to the X axis. Wall 147of bar 114 rests against inclined wall 138 by means of three returnsprings 148 (see also FIG. 9), which are inclined with respect tosurface 115 of plate 117 so as to create a downward component to keepbar 114 resting against depression 139 in plate 117.

Bar 114 is fitted by means of screws 149 with an appendix 151 of a block150; appendix 151 is fitted with a screw nut 152 engaging a screw 153connected to the shaft of a further reversible electric motor 154carried by a support 155; an appendix 156 of support 155 is fitted bymeans of screws 157 to a shoulder 158 of cavity 98; and screw 153 andthe shaft of motor 154 are parallel to top surface 115 of plate 117 andto wall 138 of depression 139.

The FIG. 9-14 device for aligning pack 1 of boards 2 operates asfollows.

The three electric motors 131, 143, 154 are controlled by circuit 66 inthe same way as motors 31, 43, 54 in FIG. 4. Pack 1 of boards 2 (FIG. 1)complete with holes 57 and 58 is first placed on fixture 113 (FIG. 9)and compressed air is fed into container 122 so that pack 1 is clampedby surface 96 of bar 116 with pin 5 inside V-shaped seat 120 of block121 and with pin 6 clamped against surface 95 of bar 114, as on the FIG.4 fixture.

Numeric control unit 87 (FIG. 6) then moves table 12 (FIG. 2) along theY axis and carriage 11 of machining heads 9 along the X axis to alignholes 57, 58 of pack 1 with respective optoelectronic sensor 59 andmemorize the error components along the X and Y axes, i.e. thecomponents of the distance between the real position of holes 57, 58 andthe reference position, in the same way as before.

Finally, the recorded X- and Y-axis components of hole 57 and the Y-axiscomponent of hole 58 are transferred to control circuit 66, which isthen enabled to control the three motors 131, 143, 154 (FIG. 10) offixture 113. Motor 143 moves intermediate member 133 parallel toinclined wall 136 of cavity 97 to produce a component of movement ofmember 133 and of block 121 according to the recorded Y-axis componentof hole 57.

In turn, motor 131 moves slide 123 and block 121 parallel to the X axisto produce a resultant movement of block 121 equal to the respectiverecorded X-axis component of hole 57. At the same time, motor 154 movesbar 114 parallel to inclined wall 138 of depression 139 to produce acomponent of movement of bar 114 according to the recorded Y-axiscomponent of hole 58, so that the centers of the two holes 57 and 58 ofpack 1 are aligned perfectly with the respective reference positions.

The method of aligning a workpiece or pack 1 on respective fixture 13,113 (FIGS. 4 and 10) therefore comprises the steps of providing pack 1with two locating elements 57, 58; fixing pack 1 to fixture 13, 113;determining the distances between the real positions of locatingelements 57, 58 and two predetermined reference positions; and shiftingpack 1 on fixture 13, 113 to eliminate said distances. Needless to say,the movements effected by motors 31, 43, 54 or 131, 143, 154 arenormally extremely small to simply correct any positioning errors due toinevitable mechanical tolerances of the component parts.

The advantages, as compared with the known state of the art, of the pack1 aligning device will be clear from the foregoing description. Inparticular, fixture 13, 113 and/or pins 5, 6 may be formed lessaccurately, thus reducing production cost. Moreover, any mechanicalinaccuracy of fixture 13, 113 and/or pins 5, 6 of packs 1 may be easilycorrected, thus eliminating rejects. And finally, in the FIG. 10variation, springs 126, 135 and 148 provide for dispensing withprismatic guides for block 121 and bar 114.

Clearly, changes may be made to the method and device as describedherein without, however, departing from the scope of the accompanyingclaims. For example, in the case illustrated, motors 31, 43, 54 or 131,143, 154 of each fixture 13 or 113 may be controlled by a correspondingcontrol circuit 66, so that the two groups of circuits 66 are enabledalternately to receive the data memorized by circuits 61.

Machine tool 8 may feature only one row of machining heads 9 and one rowof fixtures 13, 113, in which case, only one aligning cycle issufficient. The fixture 13, 113 may be applied to a machine tool havingonly one machining head and/or a table carrying one or more packfixtures; and fixture 13, 113 may be oriented with gap 19, 119 parallelto the Y axis.

Bar 16, 116 of fixture 13, 113 may be moved by a mechanism, as opposedto tubular container 22, 122; pin 5 may be clamped to block 21, 121 by aseparate mechanism; one or more of motors 31, 43, 54, 131, 143, 154 maybe replaced by linear actuators of any type; the means for transmittingmotion to block 21, 121 and to bar 14, 114 may differ from thosedescribed; and, finally, the aligning device may be used for aligningany type of workpiece on a machine tool.

What is claimed is:
 1. A device for automatically aligning a workpieceon a table of a machine tool, wherein the workpiece is fitting to aclamping fixture carried by said table to be machined by a machininghead, said head and said table being movable with respect to each otheralong two coordinate axes, said workpiece being defined by a pack ofprinted circuit boards, each said pack having two locating pins and twolocating elements, said fixture being provided with a locating memberfor clamping one of said pins, and with a pair of bars, one of said barsbeing movable towards the other bar to clamp one of said pins, saidlocating member and one of said bars being movable on said fixture bymeans of actuators, said machining head carrying an optoelectronicsensor for determining the real position of each of said elements, acontrol circuit being responsive to said sensor for controlling saidactuators so as to align said real position of said pack to apredetermined position, wherein said locating member is carried by aslide movable along one of said coordinate axes X, Y; said slide beingguided by a shift member movable along a first inclined surface withrespect to said coordinate axes X, Y; a first of said bars being movablealong a second inclined surface with respect to said coordinate axes X,Y; and said inclined surfaces determining a component of movement ofsaid locating member and said first bar along the other of saidcoordinate axes X, Y.
 2. A device as claimed in claim 1, wherein theother (116) of said bars is movable parallel to itself to engage saidpins (5, 6) against said locating member (121) and said first bar (114);characterized in that said first inclined surface (136) is parallel tosaid second inclined surface (138).
 3. A device as claimed in claim 1,wherein said fixture (113) comprises two plates (117, 118) forsupporting said pack (1); characterized in that said inclined surfacesare defined by two walls (136, 138) of one of said plates (117, 118). 4.A device for automatically aligning a workpiece on a table of a machinetool, wherein the workpiece is fitted to a clamping fixture carried bysaid table to be machined by a machining head; said head and said tablebeing movable with respect to each other along two coordinate axes; saidworkpiece comprising two locating pins, said fixture being provided witha locating member for clamping one of said pins, and with a pair of barsoperable for clamping the other one of said pins; first actuator meansfor moving said locating member along said coordinate axes, secondactuator means for moving one of said bars toward the other of saidbars; a pair of locating elements provided on said workpiece, anoptoelectronic sensor for determining the real position of each of saidelements when said pins are so clamped, and a control circuit responsiveto said sensor for controlling said first and second actuator means asto align said real position with a predetermined reference position. 5.A device as claimed in claim 4, wherein said workpiece is defined by apack of printed circuit boards, each pack being provided with said twolocating pins, and wherein said locating elements are associated to saidpins and are provided at predetermined distances from the associatedpins, said sensor being carried by said machining head to determine saidreal position upon moving said table and said head along said coordinateaxes.
 6. A device as claimed in claim 5, wherein said locating elementsare defined by two holes (57, 58) adjacent to said pins (5, 6); saidsensor (59) sensing said holes (57, 58) sequentially by means of amovement along one of the two coordinate axes X, Y.
 7. A device asclaimed in claim 5, characterized in that said sensor (59) is connectedto an image processing circuit (61) for determining the real position ofthe center of each of said holes (57, 58) and the distance between saidreal position and a corresponding reference position; said imageprocessing circuit (61) also determining the components of each distancealong said coordinate axes X, Y.
 8. A device for automatically aligninga workpiece on a table of a machine tool, wherein the workpiece isfitted to a clamping fixture carried by said table and is machined by amachining head; said head and said table being movable with respect toeach other along two coordinate axes X, Y; characterized in that saidworkpiece is fitted to said fixture by means of clamping means movableon said fixture by means of actuators; a control circuit being providedto so control said actuators as to align said workpiece in apredetermined position said actuators are defined by electric motors;two of said motors moving said locating member along said two coordinateaxes; a third of said motors moving said other bar along one of saidcoordinate axes X, Y; and said control circuit being able to controloperation of said motors according to the respective distance componentsof said holes.
 9. A device as claimed in claim 8, wherein the shafts oftwo of said motors (143, 154) and the screws of the respective screw-nutscrew couplings (141, 142; 152, 153) are parallel to the direction ofmovement along said inclined surfaces (136, 138).
 10. A device asclaimed in claim 9, characterized in that two of said motors (131, 143)and the screws of the respective screw-nut screw couplings (128, 129;141, 142) for moving said locating member (121) are housed in a firstcavity (97) of one of said plates (117, 118); the other motor (154) andthe respective screw-nut screw coupling (152, 153) for moving said bar(114) being housed in a second cavity (98) of said plate (117).
 11. Adevice as claimed in claim 10, characterized in that said shift member(133) and said first bar (114) are held resting against said inclinedsurfaces (136, 138) b return springs (135, 148).
 12. A device as claimedin claim 11, characterized in that said slide (123) is also held restingagainst said shift member (133) by at least a further return spring(126).
 13. A device as claimed in claim 11, characterized in that saidcavities (97, 98) are accessible from the top of said plate (117); eachof said cavities (97, 98) being closed by a removable plate (99, 100).14. A device for automatically aligning a workpiece on a table of amachine tool, wherein the workpiece is fitted to a clamping fixturecarried by said table and is machined by a machining head; said head andsaid table being movable with respect to each other along two coordinateaxes X, Y; characterized in that said workpiece is fitted to saidfixture by means of clamping means movable on said fixture by means ofactuators; a control circuit being provided to so control said actuatorsas to align said workpiece in a predetermined position;said clampingmember comprising a locating member and a pair of bars, said locatingmember being carried by a slide movable along one of said coordinateaxes X, Y; said slide being guided by a shift member movable along afirst inclined surface with respect to said coordinate axes X, Y; afirst of said bars being movable along a second inclined surface withrespect to said coordinate axes X, Y; and said inclined surfacesdetermining a component of movement of said locating member and said baralong the other of said coordinate axes X, Y.
 15. A device forautomatically aligning a workpiece on a table of a machine tool, whereinthe workpiece is fitted to a clamping fixture carried by said table andis machined by a machining head; said head and said table being movablewith respect to each other along two coordinate axes X, Y; characterizedin that said workpiece is fitted to said fixture by means of clampingmeans movable on said fixture by means of actuators; a control circuitbeing provided to so control said actuators as to align said workpiecein a predetermined position;said actuators defined by at least twoelectric motors having shafts acting on said locating member by means ofa screw-nut-screw coupling, said shafts of said two motors and saidscrews of said respective screw nut couplings running parallel to thedirection of movement along a first incline surface with respect to saidcoordinate axis X, Y.
 16. A device for automatically aligning aworkpiece on a table of a machine tool, wherein the workpiece is fittedto a clamping fixture carried by said table to be machined by amachining head; said head and said table being movable with respect toeach other along two coordinate axes; said workpiece being defined by apack of printed circuit boards, each pack having two locating pins, saidfixture being provided with a locating member for clamping one of saidpins, and with a pair of bars; one of said bars being movable towardsthe other bar to clamp the other of said pins; said pack also comprisingtwo locating holes adjacent to said pins; said machining head carryingan optoelectronic sensor for sequentially sensing said holes by means ofa movement along one of said coordinate axes, said sensor beingconnected to an image processing circuit for determining the realposition of the center of each of said holes and the distance betweensaid real position and a corresponding reference position; said imageprocessing circuit also determining the components of each distancealong said coordinate axes; two electric motors for moving said locatingmember along said coordinate axes; a third electric motor for movingsaid other bar along one of said coordinate axes; and a control circuitadapted to control operation of said motors according to the respectivedistance components of said holes.
 17. A device as claimed in claim 16,wherein each of said two motors (31, 43; 131, 143) has a shaft acting onsaid locating member (21; 121) by means of a screw-nut screw coupling(28-29, 41-42; 128-129, 141-142); said third motor (54; 154) having ashaft acting on said other bar (14, 114) by means of a further screw-nutscrew coupling (52-53; 152-153).
 18. A device as claimed in claim 17,characterized in that each of said motors (31, 43, 54; 131, 143, 154) isa step motor; said control circuit (66) comprising, for each of saidmotors (31, 43, 54; 131, 143, 154), a controller (76-78) for controllinga number of steps of the respective motor (31, 43, 54; 131, 143, 154)corresponding to the respective component of said distances.
 19. Adevice as claimed in claim 18, characterized by comprising aprogrammable logic control circuit (63) for controlling said controlcircuit (66) and said image processing circuit (61).
 20. A device asclaimed in claim 19, wherein said machine tool (8) comprises a number ofmachining heads (9), and said table (12) comprises a correspondingnumber of fixtures (13, 113); characterized in that each of said heads(9) carries an optoelectronic sensor (59) associated with acorresponding image processing circuit (61); each fixture (13, 113)being associated with a corresponding control circuit (66); and saidlogic control circuit (63) sequentially controlling said imageprocessing circuits (61) and said control circuits (66).
 21. A device asclaimed in claim 20, wherein said machining heads (9) are arranged intwo rows, and said fixtures (13, 113) are arranged in two correspondingrows; characterized in that the heads (9) in one of said rows comprisesaid sensors (59); said table (12) being moved along a Y axis of saidcoordinate axes X, Y to align corresponding rows of packs (1)alternately with respect to said sensors (59).
 22. A device as claimedin claim 21, characterized in that a number of said control circuits(66) are enabled by said logic control circuit (63) to alternatelycontrol the motors (31, 43, 54; 131, 143, 154) in said two rows offixtures (13, 113).
 23. A fixture carried by a worktable for clamping apack of printed circuit boards, said pack comprising two locating pins,said fixture being provided with a locating member for clamping one ofsaid pins, and with a pair of bars operable for clamping the other oneof said pins; said locating member being carried by a slide movablealong one of said coordinate axes; said slide being guided by a shiftmember movable along a first inclined surface with respect to saidcoordinate axes, said inclined surfaces determining a component ofmovement of said locating member along the other of said coordinateaxes; a first of said bars being movable with respect to the other ofsaid bars along a predetermined direction.
 24. A fixture as claimed inclaim 23, wherein said first bar is movable along a second inclinedsurface with respect to said coordinate axes; said second inclinedsurface determining a component of movement of said first bar along theother of said coordinate axes.
 25. A fixture as claimed in claim 23,including a first electric motor for moving said slide along said onecoordinate axis, a second electric motor for moving said shift memberalong said first inclined surface, and a third electric motor for movingsaid first bar along said predetermined direction.